In recent years, since crude oil and natural gas resources in oil fields located on land or in so-called shallow seas having a water depth of up to around 500 meters are being depleted, development of seabed oil fields in so-called deep seas at a depth of 1000-3000 meters, for example, beneath the surface of the sea is being actively carried out. With deep sea oil fields, it is necessary to transfer crude oil or natural gas from the well head of an oil well or natural gas well which is installed on the seabed to a platform on the water surface using steel pipes referred to as flow lines and risers.
Steel pipes constituting flow lines installed deep in the sea or rises are exposed to a high internal fluid pressure applied to their interior due to the formation pressure in deep underground regions and to the effects of water pressure of the deep sea applied to their exterior when operation is stopped. Steel pipes constituting risers are additionally exposed to the effects of repeated strains applied by waves.
Flow lines are steel pipes for transport which are installed on the ground or along the contours of the seabed. Risers are steel pipes for the transportation of oil or gas which rise from the surface of the seabed to a platform on the surface of the sea. When such pipes are used in a deep sea oil fields, it is considered necessary for the wall thickness to usually be at least 30 mm, and actually thick-walled steel pipes having a wall thickness in the range of 40 mm to 50 mm are generally used. This indicates that they are used under very severe conditions.
FIG. 1 is an explanatory view schematically showing an example of an arrangement of risers and flow lines in the sea. In the figure, a well head 12 provided on the seabed 10 and a platform 14 provided on the water surface 13 immediately above it are connected by a top tension riser 16. A flow line 18 installed on the seabed and connected to an unillustrated remote well head extends to the vicinity of the platform 14. The end of the flow line 18 is connected to the platform 14 by a steel catenary riser 20 which rises from the vicinity of the platform.
The environment of use of the risers and the flow lines is very severe, and it is said that the maximum temperature is 177° C. and the maximum internal pressure is 1400 atmospheres or more. Therefore, the steel pipes used in the risers and flow lines must be able to withstand such a severe environment. A riser is also subjected to bending stress due to waves, so it must be able to also withstand such external influences.
Accordingly, a steel pipe having a high strength and high toughness is desired for use as risers and flow lines. In order to ensure reliability, seamless steel pipes rather than welded steel pipes are used in such applications.
For welded steel pipes, a technique for manufacturing a steel pipe having a strength exceeding X80 grade has already been disclosed. For example, Patent Document 1 (JP H9-41074A) discloses a steel which exceeds X100 grade (a yield strength of at least 689 MPa) set forth in API standards. A welded steel pipe is manufactured by first producing a steel plate, rolling up the steel plate, and welding the seam to form a steel pipe. In order to impart essential properties such as strength and toughness at the time of producing the steel plate, control of the microstructure has been employed by subjecting the steel sheet to thermomechanical treatment at the stage of rolling. Also in Patent Document 1, the desired properties of a steel pipe after welding are secured by performing thermomechanical treatment during hot rolling of a steel sheet in such a manner that the microstructure is controlled so as to include deformed ferrite. Accordingly, the technique disclosed in Patent Document 1 can be realized just by a rolling process to form a steel plate in which thermomechanical treatment can be easily applied by controlled rolling, and therefore it can be applied to a welded steel pipe but not to a seamless steel pipe.
In the case of seamless steel pipes, a seamless steel pipe of X80 grade has been developed recently. With seamless steel pipes, since application of the above-described technique including thermomechanical treatment which has been developed for welded steel pipes is difficult, it is basically necessary to attain the desired properties by heat treatment after pipe formation. For example, a technique for manufacturing a seamless steel pipe of X80 grade (a yield strength of at least 551 MPa) is disclosed in Patent Document 2 (JP 2001-288532A). However, as disclosed in the examples of that document, the technique is merely demonstrated for a thin-walled steel pipe (with a wall thickness of 11.0 mm) for which hardenability is inherently good. Accordingly, even if the technique disclosed therein is employed, when a seamless steel pipe with a wall thickness of around 40-50 mm which is actually used for risers or flow lines, there is a problem in that an adequate strength and toughness cannot be attained since the cooling speed at the time of hardening is slow particularly in the central portion of such a thick-walled steel pipe.